1. Field of the Invention
The present invention relates to a roll set suited for the handling, particularly screening and spreading, of different materials such as wood chips or shavings, said roll set comprising at least two parallel-mounted, rotatable rolls, whereby the interroll gaps are adapted to pass the material being handled and at least a number of the rolls are provided with a surface patterning.
2. Description of Background Art
One embodiment of the above-mentioned type of roll set is disclosed in FI Pat. No. 90,746, the apparatus described therein being intended for spreading fibrous material such as chips and the like into a mat of predetermined structure formed onto an underlying spreading conveyor of a belt-type construction. This kind of roll set may also be used in roll screens employed for screening of wood chips or shavings. Roll screening can be employed in the form of a coplanar set of multiple adjacent rolls for screening mineral particles away from chips.
Such roll sets may utilize a surface-patterned roll of the type disclosed, e.g., in FI Pat. No. 89,082. This kind of surface pattern is comprised of tapering protuberances, mostly of a pyramidal shape.
When screening fine particles, the interroll gap between two adjacent rolls can be made very narrow, whereby the surface protuberances of the rolls run close to each other. During normal running, the tips of the surface protuberances do not make a contact with each other. However, in reality it happens every now and then that a thicker particle becomes jammed in the gap thus forcing it to open. Herein, the rolls bend resultingly causing the next interroll gap to close, sometimes even down to a physical contact. When the tips of the surface protuberances meet, the surface pattern will be destroyed particularly in rolls rotating in the same direction. Typically this takes place at the middle of the roll where the roll crown is largest. Such a damaged roll must be replaced, which is a very costly operation.
It is an object of the present invention to provide a roll set capable of overcoming the drawbacks of prior-art techniques. The roll set according to the invention is characterized in that at least some of the rolls of the roll set have at least one collar ring made thereto in an annular manner essentially about the entire circumference of the roll, whereby the collar ring has an outer diameter advantageously at least as large as the outer diameter of the other surface patterning made on the roll. Advantageously, the collar ring is made on at least every second roll of the roll set, whereby the surface patternings of the any two adjacent rolls cannot mesh with each other during an anomalous operating situation, but instead the tip of the surface pattern protuberance meets the surface of the collar ring of the adjacent roll. This kind of physical contact does not cause damage to the surface patterning.
A preferred embodiment of the invention is characterized in that each roll of the roll set is provided with a collar ring that is located essentially on the middle area of the axial length of the roll so that the collar rings of any two adjacent rolls are aligned in a substantially identical location along the axial length of the roll, whereby the collar rings make a physical contact with each other during an anomalous operating situation. As all the rolls of a roll set are then made substantially identical, there is no need for manufacturing a plurality of different rolls. Moreover, this kind of embodiment assures that under a crown of the roll, the surface patterning cannot make a contact not even to the collar ring of the adjacent roll, but instead, only the aligned collar rings of adjacent rolls can contact each other, whereby no damage can occur.
Another preferred embodiment of the invention is characterized in that the width of the collar ring is about 1-50 mm, advantageously about 10 mm and its height exceeds the outer diameter of the actual surface patterning 4 of the roll 1 by about 0.0-2.0 mm, advantageously by about 0.1 mm, depending on the height of the protuberances in the roll surface patterning. Accordingly, the shape and dimensions of the collar ring may be varied in a wide range as required. In practice, it is sufficient to make the collar ring to extend in the radial direction essentially to the outer circumferential diameter of the surface pattern protuberances, since then the collar ring can prevent xe2x80x9cmeshingxe2x80x9d of the roll surface protuberances when crowning of the rolls occurs.
A still another preferred embodiment of the invention is characterized in that the collar ring has an essentially smooth outer surface. This reduces the friction between the contacting rings to a minimum.